They're fantastic as long as you have them setup properly. They come with jumpers within the crossover's with relativly simple instructions to get them to sound best in your car.
I use to have MB's and was considering another pair but heard the SPX's and didn't want my MB's anymore.
They arn't meant to be deafening loud, they don't have a huge power requirement, they are quality. If you want great quality then get them. If you want a combination between loudness and good quality then get the MB's.
Mind you i'm powering them with an mrv-F340 which is technicaly over their RMS specs by around 30 watts and they havn't distorted yet and still delivered clear sound over two 15 inch subs at 1000 watts each and also my new sub setup of two 12 inch Type X alpines with two 10 inch Type X alpine's.. 4000 watts RMS fully powered and the type x components still blow them away.
Can't fit 4 12's horizontally across my trunk. I'm downgrading from 15's and wanted atleast 12's so i had those and i had extra space because the box was WAY smaller than the volume needed for my 15's so i got a couple of 10's to fill up the void.
Four batteries, one for each MRD-m1001 and the other amps and accessories are split evenly between the batteries.
It does the job. I always figured if i'm going to spend the money might as well go all out. The next step is to build a fibreglass mounting system to put all the caps and distrobution blocks and to cover the batteries up so the setup doesn't look like crap.